514 Prof. J. G. MacGregor and Mr. E. H. Archibald on 



According to the dissociation theory, if a solution contain 

 n it n 2 gramme-equivalents of two salts capable of forming a 

 double salt, and if a lf cc 2 are the ionization coefficients and 

 fi^ j and fjbyQ 2 the molecular conductivities at infinite dilution, 

 the conductivity, if no double molecules are formed, will be 

 ^i^iMod 1 + ^2^2/^00 2- The a's and /x^ 's involved in this ex- 

 pression can, it is true, be determined rigorously only for 

 extremely dilute solutions, and only approximately for solu- 

 tions of moderate dilution. And therefore there will always 

 be a difference between the observed and calculated values of 

 the conductivity of solutions of moderate strength, even if no 

 double salt be present as such. But we may acquire informa- 

 tion as to the magnitude of the difference due to the errors 

 involved in the determination of the a's and the fi^ 'a, by 

 comparing observed and calculated values of the conductivity 

 of a sufficient number of mixtures of solutions of salts which 

 do not, according to other sources of knowledge, form double 

 salts ; and that having been done, the observation, in solutions 

 of double salts, of considerably greater differences, such as 

 would be occasioned by the presence of double molecules, 

 would establish a greater or smaller probability of their 

 presence. It should be noted that in taking this course we do 

 not base upon the dissociation theory but upon the (supposed) 

 generalization that calculability by means of the dissociation 

 theory characterizes solutions which do not contain double 

 salt. 



We already possess a certain amount of knowledge as to 

 the closeness with which the conductivity of solutions con- 

 taining two electrolytes with a common ion, but not suspected 

 of forming double salts, maybe calculated, viz. in the case of 

 solutions containing sodium and potassium chlorides *, sodium 

 and hydrogen chlorides "f, sodium and barium chlorides J, and 

 potassium and sodium sulphates §. In all these cases it has 

 been found that up to a concentration of about 1 grm.-equiv. 

 per litre the conductivity can be calculated about as accurately 

 as it has been measured. 



Although this amount of knowledge is of course not suffi- 

 cient to afford a sufficiently wide basis for the present purpose, 

 we have thought it well to find out how closely it is possible 

 to calculate the conductivity of solutions of a double salt, 



* MacGregor, Phil. Mag. (5) xli. p. 276 (1896). 

 t Mcintosh, Ibid. (5) xli. p. 510 (1896). 

 t McKay, Trans. N. S. Inst. Sci. ix. p. 321 (1897-98). 

 S Archibald, Ibid. ix. p. 291 (1897-98). 



